An optional double is a double made over an opposing preempt. The double shows a (semi-)balanced hand and good strength. It says, "Partner, I have a good hand but am not sure whether we should play or defend." It's essentially a type of "cooperative" double. Partner is expected to pull the double with a long suit, or pass otherwise.
If you play optional doubles, this will naturally affect how high you play takeout doubles. Thus you must decide when low-level doubles stop being useful for takeout, and optional (or penalty) doubles become more practical.
How high to play optional doubles is up to partnership agreement. Some pairs play it over 3-level preempts, while others only play it over 4 and 4 preempts. The latter is recommended by Bill Root and Richard Pavlicek in their book "Modern Bridge Conventions." Root and Pavlicek recommend at least 3.5 honor tricks to double. Encyclopedia of Bridge, using standard point count methods, recommends 16 points in direct seat, or 13 points in the passout seat.
After you make a responsive double, partner has the option of either bidding a good suit or passing the double for penalties. Partner should not run with a bad hand.
- Preemptive jump raises by the opponents, e.g. 1 : 4, require partnership discussion. Doubling in these auctions tends to be more takeout-oriented because the opponents have a known fit, making it more likely that your side has one too.
- Optional doubles are most useful after three-level (and higher) preempts. Weak two-bids are better defended with takeout doubles.
- In the United States, the optional double is not alertable.